1. Field of the Invention
The present invention relates to a magneto-optical information reproducing apparatus which utilizes the magneto-optical effect to reproduce information magnetically recorded on a recording medium.
2. Related Background Art
In recent years, optical memories for effecting recording and reproduction by a laser beam have been actively studied and developed for practical use as high-density recording memories. Of these, magneto-optical disks capable of erasing and re-writing have been regarded as promising with optical disks exclusively for reproduction typified by compact disks and direct read after write (DRAW) type optical disks. Magneto-optical disks are such that information is magnetically recorded by the utilization of the localized temperature size of a magnetic thin film caused by the application of a laser spot thereto and the information is reproduced by the magneto-optical effect (particularly the Kerr effect). The Kerr effect refers to the phenomenon that the plane of polarization is rotated when light is reflected by a magnetic recording medium.
The basic construction of a magneto-optical disk apparatus according to the prior art is shown in FIG. 1 of the accompanying drawings. In FIG. 1, reference numeral 1 designates a semiconductor laser, reference numeral 2 denotes a collimator lens, reference numerals 11 and 12 designate half-mirrors, reference numeral 4 denotes an objective lens, reference numeral 6 designates a magneto-optical recording medium, reference numerals 7.sub.1 and 7.sub.2 denote analyzers, reference numeral 8 designates a condensing lens, and reference numerals 9.sub.1 and 9.sub.2 denote photodetectors. The direction of P-polarization is parallel to the plane of the drawing sheet, and the direction of S-polarization is perpendicular to the plane of the drawing sheet.
Description will now be made of a case where magneto-optical information is reproduced in such apparatus. A light beam emitted from the semiconductor laser 1 as a rectilinearly polarized light in the direction of P-polarization is collimated by the collimator lens 2 and passes through the half-mirror 11. If the P-polarized component amplitude transmittance is t.sub.p and the S-polarized component amplitude transmittance is t.sub.s, .vertline.t.sub.p .vertline..sup.2 =.vertline.t.sub.s .vertline..sup.2 =0.5 in the half-mirror 11. The light beam is imaged as a minute spot on the magneto-optical recording medium 6 by the objective lens 4. When a magnetic section (pit) is pre-formed on the medium 6, as shown in FIG. 2 of the accompanying drawings, the reflected light from the medium 6 is subjected to the rotation of the plane of polarization of .+-..theta..sub.k by the Kerr effect in conformity with whether the direction of magnetization of the spot-applied area is upward or downward. Here, if the P-polarized component of the amplitude reflectance of the recording medium 6 is R and the S-polarized component is K, the following equation is established: ##EQU1## The magneto-optically modulated reflected light is again collimated by the objective lens 4 and reflected by the half-mirror 11, whereafter it is made into a convergent light beam by the condensing lens 8 and divided by the half-mirror 12, and the divided lights pass through the analyzers 7.sub.1 and 7.sub.2, respectively, and are detected as intensity-modulated light beams by the photodetectors 9.sub.1 and 9.sub.2. That is, as shown in FIG. 2, the angle of the optic axis of the analyzer with respect to the direction of P-polarization is .+-..theta..sub.A on the transmission side and the reflection side, and the light beam is analyzed as a regular projection of the amplitude thereof onto the optic axis of the analyzer.
Considering that the Kerr rotation angle is of the order of 1.degree. and that the magneto-optical modulated component is of a very minute amount, it is necessary that the azimuth angle .theta..sub.A of the optic axis of the analyzer be set to such an optimum position that the C/N (the ratio between the carrier wave and the noise) of the detection signal becomes maximum. For example, in U.S. Pat. No. 4,569,035 issued on Feb. 4, 1986, there is shown an example of an apparatus using as a photodetector an avalanche photodiode (APD) or the like having a multiplying action wherein the azimuth of the transmission axis (the optic axis) of the analyzer is optimized. On the other hand, in an apparatus using as a photodetector a PIN photodiode or the like having no multiplying action, the azimuth angle .theta..sub.A of the optic axis of the analyzer has been set to 45.degree. so that the signal light becomes maximum. When the Kerr rotation angle is .+-..theta..sub.k, if the quantity of light incident on the recording medium is I.sub.O, the quantities of light passing through the analyzers on the transmission side and the reflection side and entering the respective photodetectors are: ##EQU2## Since .theta..sub.k .about.1.degree., .vertline.R.vertline..sup.2 &gt;&gt;.vertline.K.vertline..sup.2 is established and thus, equation (2) can be expressed as follows: ##EQU3## In equation (3), the second term in the parentheses is the magneto-optical modulated component and the first term in the parentheses is the non-modulated component. These lights are photoelectrically converted by the photodetectors 9.sub.1 and 9.sub.2, respectively, and then differentially detected by a differential circuit, not shown, whereby a magneto-optical signal is obtained.
On the other hand, a magneto-optical information reproducing apparatus using a polarizing beam splitter instead of the half-mirror 11 shown in FIG. 1 to improve the C/N of the above-mentioned reproducing signal is proposed in U.S. Pat. No. 4,561,032 issued on Dec. 24, 1985. Further, an example in which the polarizing characteristic of this polarizing beam splitter is set so that C/N is maximum is disclosed in U.S. Pat. No. 4,558,440 issued on Dec. 10, 1985.
Also, U.S. Pat. No. 4,573,149 issued on Nov. 25, 1986 discloses an example using a half wavelength plate and a polarizing beam splitter (hereinafter referred to as PBS) instead of the half-mirror 12 and the analyzers 7.sub.1 and 7.sub.2. This example will hereinafter be described with reference to FIG. 3 of the accompanying drawings.
In FIG. 3, a light beam emitted from a light source (a laser diode) 27 enters a recording medium 31 via a beam splitter 24, a mirror 25 and an objective lens 26. The light beam reflected by the medium 31 then again passes to the beam splitter 24 via the objective lens 26 and the mirror 25, and is reflected toward a condensing lens 23 by the beam splitter 24. The reference numeral 33 indicates the direction of polarization when the direction of polarization when the light beam is emitted from the light source 27 is not subjected to the Kerr effect but intactly advances toward the condensing lens 23. When the light beam is reflected by a medium 31, the light beam enters the condensing lens 23 with the plane of polarization rotated by an angle of Kerr rotation .+-..theta..sub.k with respect to the direction indicated by reference numeral 33 due to the Kerr effect. The plane of polarization is rotated by 45.degree. by a half wavelength plate 34 whose optic axis is inclined by 22.5.degree. with respect to the direction indicated by reference numeral 33 and is polarized in a direction indicated by reference numeral 35. Part of the light beam thus polarized is reflected by a beam splitter 22 and received by a servo signal detecting sensor (a photoelectric conversion element) 28, and on the other hand, the light beam transmitted through the beam splitter 22 enters PBS 21. The light beam which has entered the PBS 21 is divided into two light beams polarized in directions orthogonal to each other, and these light beams are detected by magneto-optical signal detecting sensors (photoelectric conversion elements) 29 and 30. By taking the difference between the detection signals of these sensors, the information recorded on said medium is reproduced.
However, in the construction shown in FIG. 3, the number of parts is great and the azimuths of the half wavelength plate 34 and the PBS 21 must be strictly adjusted relative to the direction of polarization of the light beam and therefore, assembly has not been easy.